Ordinary liquid crystal display devices display images that correspond to video signals thereon, by controlling optical transmittance of liquid crystal cells on liquid crystal panels. A line inversion system, a column inversion system, a dot inversion system, etc., are used as methods of driving liquid crystal cells on liquid crystal panels. In the liquid crystal panel driving method using the line inversion system, polarities of data signals supplied to the liquid crystal panel are inverted depending on row lines on the liquid crystal panel, that is, depending on gate lines and depending on frames, as shown in FIGS. 1A and 1B.
FIG. 2 is a waveform diagram illustrating a correlation of a data voltage (that is, a source output) and a common voltage according to the line inversion. Referring to FIG. 2, in order to display a specific gray, a data voltage illustrated by a dotted line and a common voltage illustrated by a solid line alternate for every one of the gate lines to thus maintain a certain level of voltage difference. Here, the generated voltage is applied to the liquid crystal to control the amount of light and display gray. For example, the voltage difference between the common voltage and the data voltage is 5V to display black. In this case, since the common voltage is larger than the data voltage in the waveform section on the left portion of FIG. 2, the common voltage is 5V and the data voltage is 0V. Next, since the common voltage and the data voltage are line inverted and thus the common voltage is smaller than the data voltage in the waveform section on the central portion of FIG. 2, the common voltage is 0V and the data voltage is 5V. This line driving system is widely used in portable devices because liquid crystal driving voltages are small to cause a small amount of current consumption.
In the liquid crystal panel driving method using the column inversion system, polarities of data signals supplied to the liquid crystal panel are inverted depending on column lines on the liquid crystal panel, that is, depending on data lines and depending on frames, as shown in FIGS. 3A and 3B.
In the liquid crystal panel driving method using the dot inversion system, data signals of opposite polarities are supplied to the adjacent liquid crystal cells on the gate lines and to the adjacent liquid crystal cells on the data lines, respectively, and polarities of data signals supplied to all the liquid crystal cells on the liquid crystal panel are inverted for each frame, as shown in FIGS. 4A and 4B. In other words, in the dot inversion system, in the case that a video signal of an odd-numbered frame is displayed, as shown in FIG. 4A, data signals are supplied to the liquid crystal cells on the liquid crystal panel, respectively, in a manner that a positive polarity (+) and a negative polarity (−) appear alternately as it goes from a liquid crystal cell shown in the uppermost-leftmost corner to the right-side liquid crystal cells, and from the liquid crystal cell shown in the uppermost-leftmost corner to the downward liquid crystal cells. In addition, in the case that a video signal of an even-numbered frame is displayed, as shown in FIG. 4B polarities of data signals supplied to the respective liquid crystal cells are inverted opposite to the odd-numbered frame.
FIG. 5 is a circuit diagram illustrating an example of a gamma voltage generating unit (or a gray scale part) that generates a data voltage in a dot inversion system. Referring to FIG. 5, a gamma voltage generating unit 5 generates a gamma voltage in the outside of a drive IC of a typical liquid crystal display device (LCD) and supplies the generated gamma voltage to a resistor string portion 7. The resistor string portion 7 subdivides the gamma voltage further and indicates required gray scale. In the present embodiment, the resistor string portion 7 has been illustrated to indicate ten (10) gray levels, that is, ten (10) data voltages, but gradation may be increased according to the number of pieces of image data such as sixty-four (64) or two-hundred fifty-six (256).
The gamma voltage generating unit 5 in the dot inversion is divided into a positive area and a negative area based on the common voltage Vcom that is located in the central portion thereof. The terms “positive area” and “negative area” are defined as the positive area when the data voltage is higher than the common voltage and as the negative area when the data voltage is lower than the common voltage. If image data that is externally applied by colors such as red, green, and blue, a data voltage corresponding, to the image data is output. For example, if image data is “08h” in red, a voltage that corresponds to the “08th” in the positive or negative area of the dot inversion is read, and a voltage that occurs due to a difference between the voltage corresponding to the “08hth” and the common voltage is applied to the liquid crystal, thereby controlling the amount of light in the red region, and displaying an image. In the present embodiment, the positive-side data voltage corresponding to “08h” will be a voltage between r0 and r1, and the negative-side data voltage thereof will be a voltage between r16 and r17. As shown in FIG. 6, these two voltages are repeated to be larger or smaller than the common voltage for each dot on the basis of the common voltage, which is called a dot inversion driving system.
The gamma voltage generating unit 5 buffers a gamma voltage via a gamma buffer 5a to then be supplied to the resistor string portion 7. A plurality of gamma buffers 5a be are typically used. The gamma voltage generating unit 5 that is divided into a positive area and a negative area is symmetrical up and down on the basis of a common voltage (Vcom) buffer 5b in FIG. 5. In the embodiment of FIG. 5, a voltage Vdd that is applied to the final stage of the positive side is 12V, as an example, and a voltage that is applied to the final stage of the negative side is 0V. In addition, VGMA_P4 is 11V and VGMA_N4 is 1V. The common voltage that has been buffered by the common voltage buffer 5b between R5 and R6 of the gamma voltage generating unit 5, is connected to a common electrode of the LCD, and the magnitude of the voltage is 6V as an example. Therefore, the highest voltage that can be set at the positive side between the common voltage and the data voltage is 5V, and the highest voltage that can be set at the negative side between the common voltage and the data voltage is also 5V. The data voltage varies depending on the voltage characteristics of the liquid crystal used. In addition, a kick-back effect that occurs when driving the LCD should be considered in order to set a rigid common voltage, but in the illustrated example, the kick-back effect has not been considered to set the magnitude of the common voltage.
FIG. 6 is a waveform diagram illustrating a relationship between the common voltage and the data voltage in the dot inversion. Referring to FIG. 6, in order to apply a voltage difference between the common voltage and the data voltage, that is 5V, the data voltage is applied as 11V in the first dot where the data voltage is greater by 5V than the common voltage 6V, and applied as 1V in the second do where the data voltage is smaller by 5V than the common voltage 6V.
The dot inversion driving system requires a high voltage, and is inverted in every dot, and thus takes a lot of current consumption. On the contrary, since the dot inversion driving system has an excellent picture quality, it is mainly used in order to drive laptop (or notebook) computers, monitors or TVs.
However, since the dot inversion driving system uses direct-current (DC) voltage as the common voltage, the common voltage does not alternate like the line inversion system. Therefore, it is not possible to achieve a second purpose, for example, a purpose of using it for touch detection, by using the alternating common voltage.